Class libraries in Java

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At this stage in the book it is worth providing some vocabulary When a class A inherits methods and variables from a class B, then A is known as the subclass of B and B is known as the superclass of A Modern object-oriented programming languages such as C++, Smalltalk and, of course, Java provide extensive class libraries which contain a large number of facilities useful to the programmer One concept that is useful to know about when using a class library is that of an abstract class Such an abstract class will contain methods known as abstract methods These methods carry out no actions at all but are just place holders which are filled when a subclass is formed by means of inheritance from the abstract class In order to explain what you might think is a weird idea it is worth looking at an example of an abstract class Class A { // Instance variables for the class // Code for method 1 // Code for method 2 // Code for method 3 abstract AbsMethod { } // Code for method n } Class A is regarded as abstract because one of its methods (AbsMethod) contains no code at all and is headed by the keyword abstract You will not be able to use AbsMethod direct; however, you will be able to use AbsMethod in a subclass by employing inheritance For example, if we want a class B which inherits the methods and variables of A and provides code to implement AbsMethod then we would write: Class B extends A { // Any new instance variables // Code for method AbsMethod
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} Using the rules for inheritance, B would become a class from which objects can be created since the new code for AbsMethod would be used whenever a message involving AbsMethod is sent You may think that this is a somewhat strange way to proceed However, class libraries get much of their power from abstract classes For example, a class library may contain an abstract class which represents window objects on a screen It could be that such a class would contain abstract methods which define the way that the border is displayed: that whenever you wish to develop a new screen object which has a different border all you need to do is to extend the abstract window class with a new method which carries out the drawing of the border The Java system contains a very large class library in which some of the classes are linked via inheritance There are five main class libraries:
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The language foundation classes These classes implement low-level data types such as integer
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and contain the code for the methods which operate on these basic data types
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The I/O class library This contains classes which provide facilities for low-level input/output,
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for example for the reading of data from files
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Another window toolkit class library This contains all the classes needed for the interaction between an applet and the user It contains classes for basic HCI components such as fonts, colours, events, buttons and scroll bars This library is used in 7
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The utility class library This library contains useful facilities such as a data class, tables, vectors
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and stacks This library is introduced in 6
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The network interface class library This vitally important library contains classes which are
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used to communicate within a network They extend the facilities that are available in the I/O class library and provide ways of, for example, connecting to other interfaces such as Telnet This has been a very brief introduction to the extensive library of classes that are provided with the Java system s 6 to 9 will provide much more detail and give you plenty of examples of the use of these libraries in developing applets and Java applications
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35 Summary
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This is probably the key chapter in the book It describes a deceptively simple idea: that Java contains a mechanism known as inheritance which enables a class to use methods and instance variables from other classes This is the main mechanism for reuse in an object-oriented programming language like Java The chapter also describes an importance graphical device known as an inheritance hierarchy, often called a class hierarchy Such a device describes the relationship between classes It shows how classes are related to each other in terms of the methods and instance variables they inherit from each other An important point made in this chapter about the inheritance hierarchy is that as you proceed down it, the classes at the bottom become more and more specialized There is still much more to learn about classes However, this detail will be presented later in 5 Before doing this it is necessary to provide a tour of the programming facilities within Java This is the subject of the next chapter
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